In many aircraft, the main propulsion engines not only provide propulsion for the aircraft, but may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical and/or pneumatic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main propulsion engines may not be capable of supplying the power needed for propulsion as well as the power to drive these other rotating components. Thus, many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may also be used to start the propulsion engines.
An APU is, in most instances, a gas turbine engine that includes a combustion system, a power turbine, a compressor, and an APU controller. During operation of the APU, the compressor draws in ambient air via an inlet duct, compresses the air, and supplies compressed air to the combustion system. The combustion system, under control of the APU controller, receives a flow of fuel from a fuel source and the compressed air from the compressor, and supplies high-energy combusted gas to the power turbine, causing it to rotate. The gas is then exhausted from the APU 100 via an exhaust duct. The power turbine includes a shaft that may be used to drive a generator for supplying electrical power, and to drive its own compressor and/or an external load compressor.
An APU may be started using any one of numerous techniques. Typically, however, an electrically-powered starter motor is used to supply a starting torque to the shaft. As the shaft rotates, the compressor draws ambient air through the APU inlet door and inlet duct, compresses the air, and supplies the compressed air to the combustor. Simultaneously, the APU controller, implementing control logic, controls fuel flow into the combustor system to maintain a desired fuel/air ratio. When the APU rotational speed reaches a predetermined speed, and when the fuel/air ratio attains what is generally referred to as “light-off conditions,” the APU controller ignites the fuel/air mixture. Thereafter, the APU power turbine augments the starter motor torque. When the APU rotational speed attains a predetermined operational speed, the APU controller de-energizes the starter motor, and the APU becomes self-sustaining and accelerates itself to operational speed.
From the above, it may be understood that a successful APU start depends on the APU rotational speed and the appropriate fuel/air ratio in the combustor, which may in turn depend on the airflow through, and thus the differential air pressure across, the APU. Too little airflow through the APU can cause high APU exhaust gas temperatures, which often cause a “hung start,” and too much airflow can prevent fuel/air ignition or cause a “blow out” of the ignited fuel/air mixture. It may additionally be understood that the differential air pressure across the APU may depend on the ambient conditions during APU startup.
Although the control logic presently implemented in APUs is safe and generally reliable, it does suffer certain drawbacks. For example, the control logic does not control the differential pressure across the APU to ensure optimal starting conditions are attained. The present invention addresses at least this need.